Process for dephosphorization of pig iron particularly pig iron rich in phosphorus



United States Patent 3,376,130 PROCESS FOR DEPI-IOSPHORIZATION 0F PIG IRON PARTICULARLY PIG IRON RICH IN PHOSPHGRUS Theodor Kootz and Heinz Maas, Duisburg-Hambom, Germany, and Erik Axel Bengtsson, Domnarvet, Sweden, assignors to Stora Kopparbergs Bergslags Aktiebolag, Falun, Sweden, a Swedish company No Drawing. Filed Mar. 2, 1965, Ser. No. 436,661 Claims priority, application Germany, Mar. 3, 1964, T 25,740 1 Claim. (Cl. 75-52) Heretofore, a number of processes have been suggested for refining pig iron by blowing technically pure oxygen onto the melt from above in which dephosphorization has been obtained early in the period of decarbonization. Steps used in these processes comprise maintaining a high basic slag, the addition of ores or slag fluxing means, limiting the temperature, control of the blast jet to a low velocity against the bath surface, providing an additional mechanical movement to the bath, addition of pulverized lime to the oxygen blast jet and others.

In practice, the refining of high phosphorus pig iron is carried out in a vessel which is stationary during the refining with oxygen being blown perpendicularly onto the bath surface and the oxygen usually contains ground lime. A first phosphate slag is withdrawn when the percentage of carbon has been reduced to about 1%, at which time the percentage of phosphorus in the steels is in the range of 0.2%-O.3%. The temperature must be far above 1500 C., as, for example, about 1600 C., in order to liquidize the highly basic slag. These slags have a ratio of basicity, i.e., bases to acids, of at least 2. The final slags, which are preferably left in the refining vessel, are still more basic and usually have a ratio of basicity of 3 or more and their ferrous oxide content are high. The decarburization reaction not only has to deliver the necessary heat, but in addition imparts a movement to the bath during decarburization by the gas evolved and thereby produces a thorough circulation. Although the stirring exect of the decarburization reaction may not be necessary in principle to produce the dephosphorization reaction, in such methods where mechanical movement is imparted to the bath, as in a Kaldo furnace, it is always used. This may be due to the fact that in an endeavor to counteract high wear on the lining of the refining vessel, it is desirable to produce liquid and hot high basic slags only near the end of the refining operation, as far as possible. In all methods, moreover, the foaming of the slags plays a considerable role. For dephosphorization, it is desirable to have a foaming slag (rich in iron oxides). On the other hand, a foaming slag often splashes violently with high iron losses, because such slags usually contain many particles of iron. Furthermore, it has been observed that this undesirable foaming occurs earlier and more violently when the percentage of carbon is high and slags of high basicity are used. If it is desired to interrupt the refining operation at higher percentages of carbon (over about 0.1% C) the dephosphorization must then have been terminated. This is not easy with the above described method where the first slagging occurs when the carbon content has been reduced to about 1%, after which there is too little time and metallurgical margin available for further reduction of phosphorus. The same limitations also apply to refining low phosphorus pig iron to make steel. High basic slags used in these refining processes are aimed at having a ratio of basicity, bases to acids, of over 2 and from 2 to 5 at the end of the refining in order to obtain a good desulphuriza- 3,376,330 Patented Apr. 2., 1968 tion as well as a sufiicient dephosphorization. Also, with these prior processes of refining Where there is a strong tendency toward splashing during the decarbonization and slag forming, it is necessary to operate with two slags if the process is to be interrupted at the higher percentages of carbon. The process according to the invention is essentially different from those hitherto used as described above.

It has now been discovered, surprisingly, that it is possible to dephosphorize a high phosphorus pig iron in a converter while refining with oxygen by using slags having a degree of basicity which is as low as the basicity of slag in a basic blast furnace where the phosphorus in the ore is quantitatively reduced and then transferred to the molten iron. The ratio by weight of basic to acid slags shall be within the limits 0.8l.5, and preferably 1.0l.2. Among such basic slag materials particularly adapted for the process are CaO and MgO and also alkali metal oxides. Acid slag materials most suitable are SiO P 0 TiO A1 0 The process is particularly adapted for the refining of high phosphorus pig iron (above 1.5% P), such as pig iron for basic Bessemer. Furthermore it is necessary with this method to keep the temperature low, namely below 1500 C., preferably within the limits 1400 C. and 1450 C. This is not only because the dephosphorization will proceed better at so low a temperature, but also because at temperatures above 1500 C. such low basicity slags cannot be used in basic vessels as they tend to attack the lining and dissolve CaO and MgO therefrom.

To dephosphorize pig iron by reacting it with such slags and oxygen, it is necessary to keep the carbon almost completely out of the reaction. Otherwise the phosphorus, instead of being taken up by the slag, is

reduced out from these slags as in the 'blast furnace and returned to the iron. It has been discovered that the process of dephosphorization cannot be performed in stationary oxygen converters during refining, but instead the bath has to be mechanically moved.

This mechanical movement of the bath can be produced by impressing a rotary swinging movement upon the bath while contained in a vertical converter vessel, such as that resulting from an eccentric movement of the vessel about a vertical axis without rotation of the vessel about said axis. Such a movement to the bath can also be produced by other rhythmic movement of the converter vessel in resonance with the swinging movement of the bath.

The process according to the invention also can be carried out in refining vessels classified as rotary either about a horizontal or inclined axis during the refining.

In carrying out this process, it is not possible to add the required amount of lime in one batch at the beginning of a refining operation. The slag would then be too basic and would not liquefy. Instead the addition of lime must be made continuous or fed intermittently in such small amounts that, with due consideration to the acid formers such as .SiO and P 0 brought into the slag in accordance with the oxygen supply, no more of basic materials are present than required by the slag composition according to the invention. When fine ground lime is used the addition should take place continuously, about proportionally to the oxygen supply, and if crushed lime is used, it may be fed in accordance with the size of the pieces and the hardness of the burnt lime.

Instead of lime, other bases also may be used, such as sodium compounds.

The required maximum temperature limit is maintained by adding cooling agents, such as scrap iron, before or during the refining.

By way of example, it is possible to obtain from a pig iron containing 3.5% C, 1.8% P, 1.0% Mn, 0.3% Si a metal containing 2.8% C, 0.3% Mn, Si and 0.10.2% P.

This result has been obtained by supplying 38 kg. of lime per ton (metric) of pig iron at the beginning of a refining operation. After five minutes of blowing with oxygen a further addition of 15 kg. per ton pig iron was made and after every additional two minutes blowing time an addition of 7.5 kg. per ton pig iron was made. Scrap iron was added in an amount of 70 kg. per ton pig iron.

The slag was maintained substantially constant during the entire course of blowing at 37%-39% CaO, 1%1.5% MgO, 6%-8% SiO 25%29% P 0 1%-2% A1 0 12%15% MnO, 8%-10% FeO. The slag had low viscosity, and had a fine foam when the furnace was tilted, which collapsed quickly, did not retain any granules and could easily be removed.

If further blowing had been continued after nine minutes, according to the example described above, the carbon would have been oxidized and a portion of the phosphorus would have been reduced and returned to the metal from the slag.

If it is desired to carry on the dephosphorization further and, possibly remove manganese from the melt, it is necessary to supply additional basic materials, particularly lime, and so much acidic materials, such as SiO or TiO or A1 0 as such, or in suitable mineral binding, that a slag having the required degree of basicity according to the invention is maintained, consideration being taken of the small quantity of phosphorus from the pig iron additionally entering the slag. The P 0 content is diminished by dilution with the acid portion supplied with the lime. In this manner it is possible to reduce the content of manganese to below 0.2% and that of phosphorus to below 0.01%.

A further surprising feature of this process was that the sulphur in the pig iron also was reduced by 50-60%, for instance, from 0.040% in the pig iron to 0.015- 0.020%, although slags which are acid from a metallurgical point of view do not take up very much sulphur. Also the content of nitrogen had decreased to below 0.002%.

When the desired degree of dephosphorization has been reached, the slag can be removed and a high percentage of very easily soluble phosphoric acid can be recovered which is a highly elfective fertilizer.

For steel manufacture, the pig iron is then decarburized in the usual way after repeated small additions of lime, preferably not more than 35 kg. lime per ton pig iron, until desired end percentage of carbon is attained. During this finishing refining the phosphorus percentage decreases to below 0.020%, the percentage of sulphur to about 0.010%, the percentage of nitrogen remaining unchanged below 0.002%, or decreasing further somewhat during the decarburization. It is cooled as required. The end slag obtained is preferably left in the converter for a subsequent heat.

The advantages of this process are that the first slag,

4 very rich in phosphorus, does not cause splashing in the converter; the slag carries no iron droplets with it resulting in small iron losses, the process produces phosphoric acid which is valuable as fertilizer, and due to its high percentage of P 0 has a ready, market. Also, the iron dephospho-rized with a resulting high percentage of carbon produces a steel particularly low in phosphorus, and the interruption in the refining process to remove slag while i the percentage of carbon is high, makes it possible to obtain a harder steel product. Furthermore, the steel obtained contains very low percentages of sulphur and nitrogen and the iron losses are remarkably low.

It is also possible to refine low phosphorus pig iron with this process. While this is true generally, the process is, however, most advantageous when the percentage of phosphorus is in the higher ranges of 0.4%-l.5% P, and were slags are obtained which are poorer in P 0 and richer in other acids the lower the phosphorus percentage of pig iron.

We claim:

1. A process for dephosphorizing a bath of moltenhigh phosphorus pig iron and producing slag having a high phosphorus content for use as a fertilizer comprising the steps of blowing oxygen onto the bath and a layer of slag floating on the top thereof, adding basic slag-forming material selected from the group consisting of calcium oxide, magnesium oxide, alkali metal oxide and mixtures thereof and acid slag forming material selected from the group consisting of silicon dioxide, titanium dioxide, aluminum oxide, phosphorus pentoxide and mixtures thereof, the proportions of said basic and acidic slag-forming materials being such during the entire blowing period as to maintain a ratio of basic to acidic materials in the slag in the range of 0.8 to 1.5 by weight, maintaining the bath at a temperature below 1500 C. by the addition of cooling agents, mechanically agitating the bath to maintain uniform conditions throughout the bath and enhance the interaction between the iron and slag, continuing the blowing of the bath so long as phosphorus is transferred from the melt to the slag, and removing the slag containing .a quantity of phosphorus pentoxide required for a highly effective fertilizer and before the slag is reduced to a degree at which phosphorus is transferred from the slag back into the melt.

References Cited UNITED STATES PATENTS 2,668,759 2/ 1954- Tenenbaum 52 2,671,018 3/1954 Graef 7552 2,693,411 11/1954 Cremer 7552 2,781,256 2/ 1957 Richards 7552 2,789,046 4/1957 Speith et al. 7552 3,136,626 6/ 1964- Decamps 7552 3,172,756 3/1965 Bengtson 7552 FOREIGN PATENTS 817,786 8/1959 Great Britain. 946,964 1/1964 Great Britain.

0 BENJAMIN HENKIN, Primary Examiner. 

1. IN A PROCESS FOR DEPHOSPHORIZING A BATH OF MOLTEN HIGH PHOSPHOURS PIG IRON AND PRODUCING SLAG HAVING A HIGH PHOSPHORUS CONTENT FOR USE AS A FERTLIZER COMPRISING THE STEPS OF BLOWING OXYGEN ONTO THE BATH AND A LYER OF SLAG FLOATING ON THE TOP THEREOF, ADDING BASIC SLAG-FORMING MATERIAL SELECTED FROM THE GROUP CONSISTINGO F CALCIUM OXIDE, MAGNESIUM OXIDE, ALKALI METAL OXIDE AND MIXTURES THEREOF AND ACID SLAG FORMING MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON DIOXIDE, TITANIUM DIOXIDE, ALUMINUM OXIDE, PHOSPHORUS PENTOXIDE AND MIXTURES THEREOF, THE PROPORTIONS OF SAID BASIC AND ACIDIC SLAG-FORMING MATERIALS BEING SUCH DURING THE ENTIRE BLOWING PERIOD AS TO MAINTAIN A RATIO OF BASICTO ACIDIC MATERIAL IN THE SLAG IN THE RANGE OF 0.8 TO 1.5 BY WEIGHT, MAINTAINING THE BATH AT A TEMPERATURE BELOW 1500*C. BY THE ADDITION OF COOLING AGENTS, MECHANICALLY AGITITATING THE BATH TO MAINTAIN UNIFORM CONDITONS THROUGHOUT THE BATH AND ENHANCE THE INTERACTION BETWEEN THE IRON AND SLAG, CONTINUING THE BLOWING OF THE BATH SO LONG AS PHOSPHORUS IS TRANSFERRED FROM THE MELT TO THE SLAG, AND REMOVING THE SLAG CONTAINING A QUANTITY OF PHOSPHORUS PENTOXIDE REQUIRED FOR A HIGHLY EFFECTIVE FERTILIZER AND BEFORE THE SLAG IS REDUCED TO A DEGREE AT WHICH PHOSPHORUS IS TRANSFERRED FROM THE SLAG BACK INTO THE MELT. 